Manually set the r0 resistance value for your thermistor. This is useful if your thermistor is not in the common pre-defined models. Besides beta and r0 which are properties of your thermistor, you can also set the r1, r2 and t0 values, but those are properties of your board so they usually never have to be changed.

This is the M-code for setting the temperature then waiting for that temperature to be reached before doing anything. For example here, the value is 109 so you use M109 S50 to set this module's heater's temperature to 50 and then wait.

Maximum PWM value for the heating element. This can be from 0 to 255. 64 is a good value if driving a 12v resistor with 24v. 255 is the default and the normal value if you are using the right voltage for your heating element.

Maximum value for the I variable in the PID control. This should usually be set to about the same value as max_pwm ( as a rule of thumb, it is not actually a pwm setting ). This helps with preventing overshoot when initially heating up. If you get a strong ( >10°C ) overshoot on startup, try setting this to a value lower than max_pwm

Set the type of sensor used to read temperature. Values can be thermistor for the usual thermistor reading via ADC method, or max31855 to read values from a thermocouple over SPI. See Reading a thermocouple